Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Tachyarrhythmias generally include supraventricular tachycardia (SVT, including atrial tachycardia, AT) and ventricular tachycardia (VT). Fibrillation is a form of tachycardia further characterized by an irregular heart rhythm. In a normal heart, the sinoatrial node, the heart's predominant natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to the atria and then to the ventricles of the heart to excite the myocardial tissues. The atria and ventricles contract in the normal atrio-ventricular sequence and synchrony to result in efficient blood-pumping functions indicated by a normal hemodynamic performance. VT occurs when the electrical impulses re-enter the atria from the ventricles to form a self-sustaining conductive loop or when a natural pacemaker in a ventricle usurps control of the heart rate from the sinoatrial node. When the heart rate reaches certain levels, the ventricles contracts before they are properly filed with blood, resulting in diminished blood flow throughout the body. This condition becomes life-threatening when the brain is deprived of sufficient oxygen supply. Ventricular fibrillation (VF), in particular, stops blood flow within seconds and, if not timely and effectively treated, causes immediate death. In very few instances a heart recovers from VF without treatment.
Cardioversion and defibrillation are used to terminate most tachyarrhythmias, including AT, VT, and VF. An implantable cardioverter/defibrillator (ICD) is a cardiac rhythm management (CRM) device that delivers an electric shock to terminate a detected tachyarrhythmia episode by depolarizing the entire myocardium simultaneously and rendering it refractory.
Another type of electrical therapy for tachycardia is anti-tachycardia pacing (ATP). In ATP, the heart is competitively paced in an effort to interrupt the reentrant circuit causing the tachycardia. An exemplary ICD's includes ATP and defibrillation capabilities so that ATP is delivered to the heart when VT is detected, while a defibrillation shock is delivered when fibrillation occurs. Although cardioversion and/or defibrillation are effective in terminating tachycardia, it consumes a large amount of power and results in patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible.
In one practice, a VT with a heart rate that is not extremely high (e.g., 180 beats per minute or less) is treated with ATP therapy in order to avoid an unnecessary painful shock to the patient, and a defibrillation shock is delivered if the pacing fails to terminate the tachyarrhythmia. However, when the heart rate is considered extremely high, many physicians are reluctant to apply ATP and choose to defibrillate immediately, even though studies have showed that ATP is still effective for a substantial majority of the patients. The primary concern is that an unsuccessful ATP therapy delays the delivery of defibrillation therapy, and that delay may comprise the effectiveness of the defibrillation therapy.
For these and other reasons, there is a need for a safe and efficient way to avoid the delivery of unnecessary defibrillation shocks and the waste of energy.